Non-woven webs used as filtration media often comprise two or more kinds of fibers, each having a different average diameter that renders the non-woven web capable of filtering particles in a broad size-range. Generally, the different kinds of fibers lie in different layers of the web—for example, a filtration web comprising a layer of 0.8 and 1.5-μm diameter microfibers melt-blown onto a spun-bonded web. Such small microfibers, exposed on the top of the web, however, are fragile and disrupt even under normal handling and use. Also, fine-diameter fibers have lower individual-fiber weight, making their transport and retention in an efficient fiber stream difficult. In addition, the fine-diameter fibers tend to scatter as they issue from a melt-blowing die rather than travel as a contained stream to a collector.
Another example of multi-layer, multi-diameter non-woven webs is the so-called SMS webs, comprising a layer of spun-bonded fibers, a layer of melt-blown microfibers, and another layer of spun-bonded fibers. Such multi-layered webs are thicker and heavier and their manufacturing is complex.
Combination webs, where a stream of fibers is mixed with another stream of fibers, are known—for example, a composite web formed by introducing secondary stream of pulp fibers, staple fibers, melt-blown fibers and continuous filaments into a primary stream of melt-blown fibers, followed by hydroentangling the deposited admixture. The web is unoriented, which gives good isotropic properties. However, the art fails to teach a web comprising a coherent matrix of continuous, oriented, thermally-bonded melt-spun fibers with microfibers dispersed in them.
Similarly, small-diameter, oriented, melt-blown fibers (for example less than 1 μM diameter) to which non-oriented melt-blown fibers may be added, are known. But once again the art fails to teach a coherent matrix of bonded melt-spun fibers in which melt-blown microfibers are dispersed.
Also known are filter elements comprising a porous molded web that contains thermally-bonded, staple fibers, and non-thermally bonded, electrically charged microfibers, with the porous molded web being generally retained in its molded configuration by bonds between the staple fibers at points of fiber intersection.
A nanofiber filter media layer is typically provided along an upstream face surface of a bulk filter media including a layer of coarse fibers. The nanofibers extend parallel to the face of the bulk filter media layer and provide high-efficiency filtering of small particles in addition to the filtering of larger particles provided by the coarse filter media. The nanofibers are provided in a thin layer laid down on a supporting substrate and/or used in conjunction with protective layers in order to attain a variety of benefits, including increased efficiency, reduced initial pressure drop, cleanability, reduced filter media thickness and/or to provide an impermeability barrier to certain fluids, such as water droplets. Previous approaches demonstrate several inherent disadvantages, such as a lack of supporting substrate, nanofiber layer/substrate delamination, rapid plugging of the filter by captured contaminants, and the alignment of nanofibers parallel to the media face surface.
Thus, there is a need for non-woven filtration media that can be customized for the rigors of a particular application, particularly, applications where the mean-flow pore diameter of the filtration media is below about 2 μM.
The present invention discloses a novel process for manufacturing non-woven webs with specific fiber diameter properties, and such non-woven webs. Specifically, non-woven webs of the present invention are useful in applications such as hepa filtration that require a lowered mean-flow pore diameter, for example, below about 2 μm. By controlling the statistical parameters of the fiber diameter the present invention prepares non-woven web that will ensure such lowered mean-flow pore diameter.